A balun may be employed in a variety of electronic circuits to transform a single-ended input signal into a balanced output signal. A single-ended input signal may be defined as an electrical signal that is carried on two signal lines with one of the signal lines tied to ground. A balanced output signal may be defined as an electrical signal that is carried on three signal lines with one of the signal lines tied to ground and the remaining two signal lines carrying electrical signals of equal amplitude but opposite phase.
A balun may be implemented by forming a pair of metal coil structures in relatively close proximity to one another. For example, a balun may include a primary metal coil structure formed adjacent to a secondary coil structure with an intervening dielectric that separates the primary and secondary metal coil structures. A single-ended input signal applied to the primary metal coil structure may be used to induce an image electrical signal in the secondary metal coil structure and provide a balance output signal from the secondary metal coil structure.
It may be desirable to form a balun so that the self-inductance of its metal coil structures is maximized. For example, a balun having metal coil structures with a relatively low amount of self-inductance may place an undesirable load on an electronic circuit. In addition, metal coil structures formed on an integrated circuit die may suffer from a relatively low amount of self-inductance as a consequence of the relatively limited space in which to form a balun.
In addition, the metal coil structures in prior baluns may cause amplitude and phase errors in its output signal. For example, the asymmetry in the input signal to a balun, i.e. one input terminal connected to ground and the other to an input signal, may cause undesirable differences in the amplitudes and phases of in the output signals on its two output terminals.